Wire is often used to electrically couple various components. A number of factors affect the electrical and physical characteristics of wire. The greater the current carrying capacity required, the more metal needed, such as copper, steel, silver, gold, aluminum, brass, nickel, copper clad steel, stainless steel, or any alloys and platings thereof. Conversely, the more metal in the wire, the stiffer and less flexible the resultant single strand wire. The less flexible the wire, the greater the work hardening of the metal due to bending and the lower the flex life (i.e., service life).
To overcome this hurdle, cable designers often replace a large gage single strand wire with a multitude of smaller strands, twisted into a flexible cable. The resultant product carries the same electrical current but is easier to manipulate and has the added benefit of a longer flex life.
There are tradeoffs, however, between improved flexibility and other physical properties. For example, the stranded wire will have a serrated exterior surface. Since the wire is formed from a plurality of strands twisted together, the exterior surface does not have a continuous round surface as that of a single strand wire. This is due to the gaps (also known as interstices) between the single ends in the twisted cable. To meet the minimum insulation thickness required by the end user, and to create the necessary round shape of coated wire, more insulation must be injected into the gaps between the strands of wire on the exterior of the wire. This results in a net waste of insulation materials. Thicker insulation also serves to reduce the flexibility of the end product.
Wire made of multiple strands also has a lower elongation, and a lower yield strength than a single strand of wire with equivalent cross sectional area. This means that stranded wire pulls apart at a lower tensile force than an equivalently sized solid wire. The interstices between the strands inside a cable provide a conduit that allows moisture to wick up a cable and into electronics located at the end of the cable, which may cause corrosion.
The improved flexibility of the multiple strand wire comes at a steep price. The greatest cost of manufacturing wire typically occurs in two areas: (1) drawing the larger single strand wire down to the smaller multiple strands; and (2) twisting the multiple strands back up into cable. Current technology requires a significant investment in the purchase, installation and operation of large, capital intensive equipment. Due to the separate manufacturing operations, there are substantial productivity costs. For example, whether stranding or bunching, existing devices require the wire to be twisted as a separate manufacturing operation. Existing devices are also physically incapable of drawing multiple strands of wire, twisting them and coating them in one operation.
There are other downsides to multiple strand wire. For example, existing devices take up substantial floor space. Existing drawing devices are large, bulky and require specialized ancillary processing equipment. Since current twisting device's line speed is 10% of the other processes it therefore needs ten-fold the amount of floor space.
The existing process requires the manufacture and storage of large amounts of Work In Process (“WIP”) materials. Single strands must be stored in containers; stranded uncoated cable must also be stored in a container. The cost of WIP can be expressed in value added material stored in an inventory location. Eliminating or reducing WIP would reduce overall time from purchase order to delivery, since no time or materials would be spent to create WIP.
When all the cable has been consumed from the WIP container, or when finished goods container has been completely filled, the process must be interrupted. This represents a huge inconvenience and loss of productivity. Because WIP containers must be changed at regular intervals, and to avoid re-stringing the entire process line, the cable ends must be joined together to form the continuous length of finished product. Existing joining devices require the use of butt welders and/or brazing techniques. This generally creates a weak point in the wire that must be removed from the finished cable. Because current technology requires wire to be stored in containers between operations, there is a quantifiable and significant expense in moving WIP between storage locations and between processes. This expense is in the form of labor and equipment to move the WIP.
Another disadvantage of stranded wires is the payoff and takeup equipment required before and after each manufacturing step in the existing process. This equipment represents a significant investment in capital equipment and is responsible for a non value added increase in complexity, maintenance and equipment costs.
Because the existing drawing, stranding and extrusion operations are completely separate and unconnected, each operation therefore has discrete and unconnected manpower requirements. Wire drawing process requires perishable tooling to form and control wire outer diameter (“OD”). The smaller the diameter of the single strands, the greater number of perishable tools required. Large multi-wire drawing machines also require matched-diameter die sets of perishable tooling which comes as an added expense. Moreover, the sheer size of current technology requires an enormous operating expense.
According to one aspect, the invention provides a single strand wire with improved flexibility. The wire may be formed by a process including the steps of providing a source of single strand wire defining a longitudinal axis. The process may include the step of twisting the single strand wire in a first direction about the longitudinal axis. A longitudinal groove may be formed in the single strand wire. The wire may then be reshaped into a substantially round cross-section. The process may include the step of twisting the single strand wire in a second direction about the longitudinal axis, forming a helical groove in the outer circumferential surface of the wire body to improve flexibility.
In another aspect, the invention provides a flexible, single strand wire, which may include a solid, single strand wire body. A helical groove may be formed on an outer circumferential surface of the wire body to improve flexibility.
According to another aspect, the invention provides a stranded cable, which includes a cable body with a plurality of ductile metal strands. Typically, the strands are severed from the same single strand wire.
In yet another aspect, the invention provides a stranded cable formed by a process that includes the step of providing a source of single strand wire defining a longitudinal axis. The single strand wire is twisted in a first direction about the longitudinal axis and severed along the longitudinal axis to form a stranded cable with at least two strands. This stranded cable is then twisted in a second direction about the longitudinal axis.
Additional features and advantages of the invention will become apparent to those skilled in the art upon consideration of the following detailed description of the illustrated embodiment exemplifying the best mode of carrying out the invention as presently perceived.